The objective of this study was to investigate K, Ca, and Mg immobilization in the stem of species typical of the hardwood forest of southern Quebec. The species examined included American basswood, sugar maple, and white ash from a rich site, and American beech, red oak, and red maple from a poor site. Firstly, rates of immobilization were evaluated over a time span of 40 years. Higher rates of immobilization in the wood only were generally observed on the rich site. Sugar maple immobilized Ca at an elevated rate during 1978--1997, in comparison to the other species on site. In the case of Ca for white ash, and Ca and Mg for red oak, a low wood element concentration and a high rate of growth was associated with a low rate of immobilization. Mean annual immobilization rates on a whole stem basis were also determined for the life of the tree. These results suggest that American basswood and red oak immobilize significant amounts of Mg and Ca in their bark, respectively. / In a second study, concentrations of K, Ca, and Mg across the radial section of the stem (heartwood, transitional, sapwood, and bark) were determined. In a majority of cases, the bark was highest in concentration of base cations. Whereas, for the woody portion of the stem, concentrations were generally highest in the heartwood.

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The presence of mosses in black spruce forests is known to have an important impact on the availability and abundance of nutrients in this ecosystem. Mosses contribute to long-term accumulation of organic matter and storage of nutrients as well as to short-term nutrient release. In the boreal forest of northwestern Quebec, the effect of mosses on nutrient cycling was examined within the framework of a chronosequence ranging from 25 to 300 years of age. Laboratory and buried bag incubations, total nutrient digests, respirometric and root abundance measurements as well as moisture and temperature measurements were done to characterize the nutritional status of the organic matter profiles. In general, no change in moss accumulation, or nutrient storage or availability across the chronosequence could be detected. There were, however, differences between feather mosses and Sphagnum mosses, the latter having significantly lower levels of nutrients than the former in terms of mineralizable nitrogen and total carbon, nitrogen, phosphorus, magnesium and potassium content. The nutritional profile of moss cores showed different horizons, reflecting differences in organic matter quality down the organic layer. More specifically, nitrogen availability on a concentration basis as well as root abundance decreased with depth. Forest floor temperature seemed to confine the most biologically active horizon, referred to as the active layer, to a shallow depth, however neither temperature nor moisture seemed to explain the nutritional differences between feather mosses and Sphagnum mosses. These results may lead to practical consequences in that they show a clear distinction between the effects of feather mosses and Sphagnum mosses in nutrient cycling, suggesting that moss cover, might be useful as an indicator of site nutritional status. The results also show that accumulation of nutrients in organic surface horizons, as has been observed elsewhere, does not appear to occur in blac

No description available.

No description available.

The objective of this thesis was to validate the dynamic model SAFE (Soil Acidification in Forested Ecosystems) in a small deciduous watershed of southern Quebec. SAFE could then be used to: (1) identify which processes are governing acidification, and (2) assess the rates of acidification according to various forest conditions. / Soil and soil solution chemistry between unburned and burned zones following fire disturbance seventy-five years ago was examined within the watershed. Results showed two major, statistically significant, differences: (1) higher base status, and (2) lower soil solution N in the burned zone. High quality leaf litter of aspen and birch (burned zone) relative to that of sugar and red maple (unburned zone) has contributed to the enrichment of base cations in the forest floor. The enrichment of the forest floor did not however impoverish the B horizon as seen in other studies. Rather, fire enriched the soil in base cations and buffered the effect of forest regrowth in the B horizon. / The MAKEDEP model was used to reconstruct the time-series input files needed to run SAFE. In MAKEDEP, the availability of N determines tree growth which in turn, affects most of the processes involved in nutrient cycling. Regressions of measured deposition at the Hubbard Brook Experimental Forest and that of simulated deposition at the study site suggest MAKEDEP is suitable to model the deposition trends of all elements except Na. / SAFE was calibrated for the unburned and burned conditions at the study site. Fire disturbance and forest regrowth have produced different soil chemical composition within the zones as discussed above. SAFE was therefore validated at the study site as a function of its ability to reproduce soil chemistry under unburned and burned conditions. The simulated soil chemistry was in close agreement with the measured unburned soil conditions, but some processes would have to be clarified or accounted for with greater accurately, e.g., biological N fixation and N immobilization by myccorhizal fungi, to reproduce more accurately the measured burned soil chemistry. Simulated soil chemistry in the unburned zone reinforced nevertheless the conclusions of a few historical soil chemistry studies supporting the hypothesis that acid-sensitive forest sites of the United States underwent significant acidification during 1930--1980 during major input of acidity from the atmosphere. Model output suggests that cation nutrient deficiencies could occur in the long-term, but future Al phytotoxic responses are unlikely to occur due to a relatively high projected pH. (Abstract shortened by UMI.)